Removal of unwanted foreign materials, process residues, particles, and the like (hereinafter be referred to as contaminant particles or CPs) from the surface of substrates is extremely important for yield and reliability of components in the manufacture of microelectronic products. The term substrate as used herein includes semiconductor wafers, ceramic substrates, printed circuit boards, display panels, displays, disks and other assemblies. Contaminant particles in microelectronics manufacturing has been recognized as originating from ambient including tools, operators and processes which are used in the manufacturing process. In spite of the use of manufacturing rooms equipped with filters (HEPA filters) and blowers to remove airborne particles, use of clean room garments and processes using clean chemicals and equipment designed for low particulate generation, it has heretofore not been possible to eliminate the build up of a certain amount of CPs on substrates during processing, handling and storage. It is therefore a general practice to clean the substrates at certain process intervals to reduce the amount of CPs. Wet cleaning is most widely used, and employs de-ionized water or an alcohol (IPA- iso propyl alcohol) for rinsing/spraying the substrates multiple times to dislodge (overcome adhesion) the particles. A first step in the removal of CPs is to overcome their adhesion to the surface of a substrate.
The adhesion of CPs to the substrate can vary depending on the CP size and the nature of the adhesion forces involved, i.e. whether the force is chemical or physical in origin (M. B. Ranade, in Aerosol Science and Technology, 7, (1987) p161-176). Chemical forces involve interaction /exchange of valence or core electrons between the CP and the substrate. Physical adhesion forces, most common in the adhesion of CPs to substrates in microelectronics, are usually of either van der Wall type exchange or electrostatic forces. These forces can be substantial, and their strength can be several times that of gravitational forces. When the adhesion forces are predominantly electrostatic in nature, selected chemical solutions are often used to neutralize surface charges and thus diminish the electrostatic adhesion. Chemically bound CPs have higher adhesion energies, comparable to binding energies of solids. They are difficult to dislodge by most cleaning techniques and may need to be sputtered away or dissolved by a solvent. The physically bound CPs, in general, can be dislodged by momentum transfer from impinging gases, fluids or solids, a concept used in spray, ultrasonic and aerosol cleaning. Of the processes utilizing momentum transfer, aerosol cleaning has been found most effective in the removal of CPs. This has been described in U.S. Pat. No. 5,062,898. The aerosol used is predominantly solid particles mixed with some gas or liquid. A highly pressurized, precooled gas/liquid is allowed to expand through an orifice, further cooling the gas/liquid due to Joule-Thompson effect, which results in the formation of condensed solid particles. The mixture of solid and gas/liquid particles in the jet is used to blast the susbtrate at high velocities.
Removal of smaller particles is usually more difficult because the adhesion force decreases only linearly with the dimension of the particle, whereas the force that can be applied to overcome adhesion decreases with the area (square) or volume (cube) of the particle. A commonly occurring applied force is gravity, which varies with the volume of the particle. Therefore, applied forces decrease more rapidly with particle size compared to adhesion forces, thereby making the task of removing particles very difficult. Further, the formation of a boundary layer during blasting/cleaning operation, makes coupling of the applied force to the particle less effective. When a substrate is blasted by a medium containing a fluid or in the presence of a fluid forming ambient, a boundary layer is formed on the substrate surface. Usually the boundary layer is made of the impinging gas/liquid or gaseous/liquid layer present on the surface of the substrate. When the CP size is of the order of the thickness of the boundary layer, as the case for sub-micron particles, the boundary layer effectively shields the sub-micron particles from the impact of the high pressure spray of the gas or liquid. However, in aerosol cleaning, the solid/hard particles (frozen particles) in the aerosol, are able to penetrate through the surface boundary layer of the substrate, and impinge on the CP. The dynamic impact due to the impinging of the particles can exert a very substantial force overcoming the adhesion of the CP. This force is the first order time derivative of the impact energy. The solid particles in the high pressure aerosol, are propelled with speed near the velocity of sound and momentum transfer between the aerosol particles and FM is completed within a small fraction of a second, thereby impressing a substantial momentary force on the CP to overcome its adhesion.
Some aspects of the use of aerosol cleaning are described in the following US Patents. The features described generally cover composition, apparatus and the cleaning application.
U.S. Pat. No. 4,617,064 describes a particle-blast cleaning apparatus, wherein CO.sub.2 pellets are propelled by high pressure CO.sub.2 carrier gas and a nozzle is manually directed over the cleaning objects.
U.S. Pat. No. 4,747,421 describes an apparatus for removing photo-resist film from a substrate employing fine CO.sub.2 particles. The substrate is affixed to a slidable rod, and moved with respect to the stationary nozzle by a sliding and turning motion. This patent also notes that a 45 degree jet to substrate orientation was much more effective than a 90 degree orientation.
U.S. Pat. No. 4,974,375 describes an apparatus for cleaning of substrates using a jet of ice particles. The substrate is held at a near vertical angle by a supporting table, which is rotated by a motor so that the ice particles hit uniformly over the whole surface of the solid, and the solid surface can be cleaned uniformly.
U.S. Pat. No. 5,035,750 describes the cleaning of semiconductor wafers by blasting the surface with frozen ice particles propelled by a pressurized nitrogen gas. The wafers are moved by conveyor belt.
U.S. Pat. No. 5,062,898 describes the use of an argon aerosol jet for cleaning microelectronic parts.
U.S. Pat. No. 5,147,466 describes a method of cleaning a surface by blasting fine frozen particles against the surface, the hardness of the frozen particles being selected by use of different aerosol materials.
U.S. Ser. No. 07/958,417, filed Oct. 9, 1992, allowed Dec. 14, 1992, relates to an apparatus for cleaning with a cryogenic aerosol. The apparatus described contains: nozzle design, a cleaning chamber housing the nozzle, a substrate holder which supports the wafer and moves the wafer with respect to the aerosol spray in a linear and curvilinear manner to spray over all of the substrate surface, and, a carrier gas to help sweep out the dislodged particles from re-landing on a different part of the substrate. Thus, the features described, are focussed toward effective dislodging and removal of CPs from the vicinity of the substrate being cleaned. The carrier gas feature of the apparatus is expensive to design and operate.
It is therefore desirable to overcome some of the deficiencies in the prior art aerosol cleaning apparatus and process. One of the short coming of prior aerosol cleaning apparatus is the inability to remove effectively the loosened particles from the aerosol spray on a substrate, requiring the aerosol spray to overcome the adhesion of CP to substrate, but also impart sufficient momentum so that CP can clear the susbtrate being cleaned. Use of nitrogen carrier gas to carry away the air borne CPs is effective only with very small air borne CPs, and further requires that the particle be ejected off the substrate first. The use of nitrogen at high flow rates can be expensive. Another short coming of prior art aerosol cleaning process is the need to make multiple spray passes of the substrate linearly back and forth in order to sweep the CPs from one end of the substrate to the other. This is time consuming and lowers the productivity of aerosol cleaning. A further general deficiency of prior aerosol apparatus is the need to use part of the aerosol particle momentum to impart a horizontal velocity on the CPs. This means that aerosol bombardment energy can not be optimized for overcoming CP adhesion alone.